1. Field of the Invention
This invention relates generally to apparatus and methods for removing sorbates from sorbents and more specifically to such methods and apparatus which remove the sorbates based on different boiling points (BP) and/or different levels of desorption activation energy (DAE).
2. Discussion of Relevant Art
In the process of heating various substances, chemicals are often released which must be collected and disposed of. This is particularly true with respect to volatile organic chemicals (VOCs) which are often toxic or otherwise dangerous. In one such process, wood is dried by heating. In this process, water is given off along with volatile organic chemicals including methanol, formaldehyde and terpines. In this process, the chemicals are typically adsorbed onto a resin-base material such as Optipore (a trademark of Dow Chemical) resin, carbonaceous resin or even activated carbon. Other processes rely on absorption to withdraw VOCs and other chemicals. Given that the current invention relates to processes involving absorption as well as adsorption, the chemicals involved, such as VOCs, will be referred to as sorbates, and the attraction media, such as the Optipore resin, will be referred to as sorbents. In these processes, the sorbents approach their maximum level of sorption, at which point, the sorbates are fully adsorbed onto or absorbed into the sorbents.
The spent sorbents are then processed to remove the sorbates so that the sorbents can be reused. In the past, the spent sorbents have been introduced to a single stage desorber where hot stripping gases have been introduced to the sorbents at a level sufficient to remove substantially all of the sorbates from the sorbents. The temperature of the gas has been sufficient to volatilize all of the VOCs simultaneously. The effluent vapor including all of the VOCs has typically been burned off. The cost of this process has been derived primarily from the cost of heating the stripping gas. Since all of the VOCs must be volatilized, the degree of heat required is that associated with the VOC having the highest boiling point (BP) or desorption activation energy (DAE). Attempts to economize and otherwise facilitate these processes have included various methods for introducing the spent sorbents to the desorber. As a result, the single stage process has been applied to fluidized beds, moving beds and even packed beds or batch processes.
These deficiencies of the past are overcome with the present invention which appreciates that each of the VOCs has a different boiling point (BP) and/or a different level of desorption activation energy (DAE). A dual stage desorber is provided with a primary stage which may include a fluidized bed, a moving bed or a packed bed. A hot stripping gas is introduced to the spent sorbents at a temperature sufficient to volatilize only those VOCs having the lowest BP and/or the lowest DAE components. In the exemplary process for drying wood, these components with the lowest BP or DAE may include methanol, formaldehyde and/or water, but not the terpines. For destructive application, the removed vapors can be burned or oxidized. A heat exchanger can also be used to recover some of the thermal energy to heat the stripping gas. For recovery applications, the removed vapors can be directed to a cooling system to be condensed.
In a second stage of the dual-stage desorber, the sorbates or VOCs having the highest BP and/or highest DAE, such as the terpines, can be removed from the sorbents together with any residual lower BP and/or lower DAE components. In this removal step, microwave or infrared energy can be used with a sweeping gas, such as nitrogen, to provide the desorption energy. Alternatively, or perhaps in combination, a hot fluid can be directed through the bed at a temperature substantially higher than that applied in the first stage. The resulting vapors can be directed through a cooling system for recovery or further fractionation. For example, the vapors can be distilled to a liquid state. This can be particularly advantageous to the economies of the process, as these VOCs in the liquid state have a market value and can be reused or resold to offset some of the costs of the process.
The resulting process offers several advantages both technologically and economically. From a technical standpoint, the sorbates can be separated so that they can be dealt with separately. Those having future value can be reused.
From an economic standpoint, the total volume of the sorbates is reduced in the first stage where the operation has a lower cost per unit weight. In the second stage where the cost per unit weight is greater, the overall weight has been reduced so that the overall cost is significantly less. Notwithstanding these reductions in the cost of operation, the separation of VOCs enables those having future value to be separated and reused or resold.
These and other features and advantages of the invention will become more apparent with the description of preferred embodiments in reference to the associated drawings.